Multi-Process/Multi-Threaded C Server reads null-string from socket - c

Question

I'm working on a Server in C using a Multi-Process / Multi-Threaded architecture. At the start I've a main process that forks 10 different processes. Each process creates a pool of threads. Each process is, also, the controller of its pool (creates new thread / extends the pool when needed).
The processes also listen on the socket (whose access is controlled by the main process with an unnamed semaphore in a shared memory area) and pass the socket file descriptor to one of the thread in their pool when a connection is acquired. The chosen thread (which is awakened by a pthread_cond_signal) reads from the socket with a recv using the option MSG_DONTWAIT.
ssize_t readn, writen;
size_t nleft;
char *buff;
buff = malloc(sizeof(char) * BUFF_SIZE);
if (buff == NULL) {
perror("malloc");
return EXIT_FAILURE;
}
char *ptr = buff;
/*if (fcntl(connsd, F_SETFL, O_NONBLOCK) == -1) { // set to non-blocking
perror("fcntl");
exit(EXIT_FAILURE);
}*/
errno = 0;
nleft = BUFF_SIZE;
while(nleft > 0) {
if ((readn = recv(connsd, ptr, nleft, MSG_DONTWAIT)) < 0) {
if (errno == EAGAIN || errno == EWOULDBLOCK) {
*ptr = '\0';
break;
}
else if (errno == EINTR)
readn = 0;
else {
perror("read");
return EXIT_FAILURE;
}
}
else if (readn == 0)
break;
if (buff[strlen(buff)-1] == '\0') {
break;
}
nleft -= readn;
ptr += readn;
}
The problem is that sometimes, when I try to connect to the server using Chrome (or Firefox) 3 threads seems to receive the HTTP Request but each one of them simply closes the connection because of this portion of code.
if (buff[strlen(buff)-1] != '\0') {
printf("%s\n",buff);
fflush(stdout);
buff[strlen(buff)-1] = '\0';
}
errno = 0;
if (strlen(buff) < 1) {
perror("No string");
if (shutdown(connsd,SHUT_RDWR) < 0) {
perror("shutdown");
return EXIT_FAILURE;
}
if (close(connsd) < 0) {
perror("close");
return EXIT_FAILURE;
}
return EXIT_FAILURE;
}
Other times we have 3 threads with different behaviours: one of them receives and reads from the socket the first HTTP Request (GET / HTTP/1.1). The second one is empty (request received (because it is awakened (?)) no string read). The third one receives and reads another HTTP Request (GET /favicon.ico HTTP/1.1).
Where is the problem that cause these behaviours? I can add other portions of code if needed.
Thank you very much for your time.